Re: [Qemu-devel] [PATCH 0/3] v4 Decouple block device removal from device removal

[Daniel P. Berrange]

On Mon, Nov 08, 2010 at 06:56:02PM +0200, Michael S. Tsirkin wrote:
> On Mon, Nov 08, 2010 at 08:02:50AM -0600, Ryan Harper wrote:
> > * Markus Armbruster <address@hidden> [2010-11-08 06:04]:
> > > "Michael S. Tsirkin" <address@hidden> writes:
> > > >> Here's how the various objects are connected to each other:
> > > >> 
> > > >>                contains
> > > >> drivelist    -----------> DriveInfo
> > > >>                                 |
> > > >>                                 | .bdrv
> > > >>                                 | .id == .bdrv->device_name
> > > >>                                 |
> > > >>                contains         V
> > > >> bdrv_states  -----------> BlockDriverState
> > > >>                              |   ^
> > > >>                        .peer |   |
> > > >>                              |   |                          host part
> > > >> -----------------------------|---|-----------------------------------
> > > >>                              |   |                         guest part
> > > >>                              |   | property "drive"
> > > >>                              v   |
> > > >>                           DeviceState
> > > >> 
> > > >> To disconnect host from guest part, you need to cut both pointers.  To
> > > >> delete the host part, you need to delete both objects, BlockDriverState
> > > >> and DriveInfo.
> > > >
> > > >
> > > > If we remove DriveInfo, how can management later detect that guest part
> > > > was deleted?
> > > 
> > > Directly: check whether the qdev is gone.
> > > 
> > > I don't know how to check that indirectly, via DriveInfo.
> > > 
> > > >              If you want symmetry with netdev, it's possible to keep a
> > > > shell of BlockDriverState/DriveInfo around (solving dangling pointer
> > > > problems).
> > > 
> > > netdev_del deletes the host network part:
> > > 
> > >     (qemu) info network
> > >     Devices not on any VLAN:
> > >       net.0: net=10.0.2.0, restricted=n peer=nic.0
> > >       nic.0: model=virtio-net-pci,macaddr=52:54:00:12:34:56 peer=net.0
> > >     (qemu) netdev_del net.0
> > >     (qemu) info network
> > >     Devices not on any VLAN:
> > >       nic.0: model=virtio-net-pci,macaddr=52:54:00:12:34:56 peer=net.0
> > > 
> > > It leaves around the VLAN object.  Since qdev property points to that,
> > > it doesn't dangle.
> > > 
> > > In my opinion, drive_del should make the drive vanish from "info block",
> > 
> > Yeah; that's the right thing to do here.  Let me respin the patch with
> > the name change and the additional work to fix up the pointers and
> > ensure that we don't see the drive in info block.
> 
> Daniel, I'd like your input here: can you live with
> device diappearing from info block and parsing
> qdev tree info to figure out whether device is really gone?

We don't use info block for anything. Having to parse the full qdev tree
to determine if a single device is gone seems rather tedious. It would
be better if query-qdev took an optional argument, which is the name
of the device to root the tree at. Then checking whether a device
named 'foo' is gone just means running 'query-qdev foo' and seeing if
that returns an error about the device not existing, then we know it
has gone. No need to parse anything. Being able to query the qdev data
for a single device, or sub-tree of devices seems useful in its own
right.

Regards,
Daniel
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